For wing covering of airplanes linen had formerly been thought necessary, but the supply of linen was practically exhausted and there was none for the airplanes we must build. The Western Allies had been experimenting upon cotton materials for some time but had thus far produced no fabric possessing the necessary strength. A substitute for linen for the wing coverings of our airplanes was an absolute necessity. American chemists and members of the Signal Corps had already been working upon a series of experiments upon cotton fabrics and they presently devised a method of treatment that made them as good as linen for this purpose and thereafter this substitute was used by both our war associates and ourselves. When the armistice was signed 1,200,000 yards of this material were being manufactured and treated per month. Castor oil was necessary for the lubrication of airplane engines, but the world’s available supply was barely sufficient for the planes of our war associates and we would have to grow the beans and make the oil for the engines of our own planes. Castor beans for seed were rushed from India and planted by the thousand acres and machinery installed for crushing the beans and refining the oil. In the meantime, chemical experiments were being made for the purpose of discovering or devising a substitute. They were finally successful and an oil was produced that was equally good for all except the rotary type of engine. Not only had the production of airplanes and engines to be provided for but a great variety of accessories of which the country had none was equally necessary. The aviators needed special clothing and equipment; for the battle planes there had to be mechanism synchronizing their machine-gun and propeller action, new kinds of ammunition, bombs and bomb accessories specialized for air combat; planes of all kinds had to be equipped with many kinds of gauges, meters and other instruments requiring the most delicate and exact work in their manufacture, and necessary also were cameras for air use and camera guns for training purposes. The manufacture of all these and many other accessories had to be instituted and rushed forward and, because of the shortage in skilled labor and the need for it in so many kinds of war production at the same time, workers had frequently to be trained for the making of them. At the end of hostilities between three and four hundred manufacturing concerns, employing over 200,000 skilled workers, were supplying the various needs of this highly specialized branch of warfare.